In an electronic circuit system, random access memory (RAM) is an essential component. Conventional types of random access memory include static random access memory (SRAM) and dynamic random access memory (DRAM). However, the data stored in SRAM or DRAM disappears after the system power supply shuts down. Therefore, for applications that require the data to be kept after the power supply shuts down, a memory device that can keep the stored data after the power supply stops supplying power is desired. Nonvolatile memory (NVM) is a device satisfying such applications.
Currently, the types of nonvolatile memory that have been developed include flash memory, eFuse, magnetoresistive random access memory (MRAM), ferroelectric random-access memory, phase change memory (PCM) and resistive random-access memory (RRAM), etc. Those kinds of memory device can retain the stored data after a power supply shuts down. Especially, the resistive random access memory (referred to as resistive-type memory herein) is a type of nonvolatile memory which the industry is actively developing. The resistive-type memory has advantages in low operational voltage, short writing time, long data retention period, simple structure, smaller circuit area, etc., and would be a type of memory device with a great application potential.
Despite the advantages of resistive-type memory mentioned above, some data writing problems have yet to be overcome. First, in a conventional writing method, the voltage drop across a resistive-type memory cell would change during a writing process, and the voltage drop across the memory cell may be so large as to overstress the memory cell and thus damage the memory cell or deteriorate the reliability of the memory cell.
Secondly, due to manufacturing processes or other factors, some of the resistive-type memory cells are fast writing memory cells (fast cell). These fast writing memory cells can be written faster than other memory cells. That is, under the same writing conditions, the fast writing memory cells complete a writing process faster than normal memory cells. In other words, the fast writing memory cells take less time to complete the writing. However, in a conventional writing method, the fast writing memory cells are not treated separately from normal memory cells, so the fast writing memory cells are subject to the same amount of writing time as normal memory cells. This may make the fast writing memory cells subject to superfluous stressing time. Namely, after completing the writing, the fast writing memory cells are subject to superfluous time of writing conditions, leading to extra stress. This may also damage the fast writing memory cells. Especially, these issues become more and more severe as semiconductor manufacturing technology advances. Therefore, an excellent method for writing resistive-type memory is desired to overcome the above issues.
In view of the issues of overstressing the fast writing memory cells and superfluous time of stressing the fast writing memory cells, this invention provides methods and circuits for self-terminated writing with quasi-constant voltage drop across resistive-type memory to solve the resistive-type memory related writing issues. The detailed description and advantages of the present invention are further set forth in the Summary and Detail Description sections. It is noted that the description set forth below is for the purpose of better understanding the present invention, not for limiting the scope of the present invention.